Shaykh Uthaymeen said: “We have to call these people [those] who claim to be Muslim but who commit acts of bidah which may constitute kufr (disbelief) or may be less than that – to the truth, by explaining the truth without being hostile or condemning what they are doing [in an abusive/insulting manner or an aggressive manner].
Allaah says, ‘And insult not those whom they (disbelievers) worship besides Allaah, lest they insult Allaah wrongfully without knowledge.’ [al-An’aam 6:108]
If we find out that they are stubborn and arrogant, then we should point out their falsehood [in the best of manners]…
With regard to boycotting them, that depends upon the bidah.
If it is a bidah which constitutes kufr, then it is obligatory to boycott the person who does it.
If it is of a lesser degree than that, then it is essential to examine the situation further.
If something may be achieved by boycotting the person, then we do it; if no purpose will be served by it, or if it will only make him more disobedient and arrogant, then we should avoid doing that, because whatever serves no purpose, it is better not to do it.
Also in principle it is haraam to boycott a believer, because the Prophet said, “It is not permissible for a man to forsake [not speak to] his brother for more than three [days].””
Ref: Slightly Adapted from a verdict found in Majmoo’ Fataawa Ibn Uthaymeen, vol. 2
Posted by Abdul Kareem Ibn Ozzie